1. Field of the Invention
The present invention relates generally to the manufacture of metal tubes for use in nuclear fuel rods and, more particularly, is concerned with an induction annealing method and apparatus particularly useful in conjunction with cold-pilgering manufacture of thin-walled metal tubes.
2. Description of the Prior Art
Metal tubes used in the manufacture of nuclear fuel rods are fabricated by multiple passes in a conventional cold-pilgering reduction process. In each pass of the cold-pilgering reduction process, a tube is advanced over a stationary mandrel and simultaneously compressed using two opposing roller dies resulting in the reduction of the cross-sectional area and in elongation of the tube. After fabrication by multiple pilgering passes, the tubes are subjected to a variety of processing steps including annealing, etching, rinsing, drying, carbide grit blasting, polishing, cleaning and inspecting of the tubes.
As practiced heretofore, metal tube annealing has been carried out generally on a batch basis, meaning that tubes are collected and annealed in lot size bundles, usually six hundred to one thousand to a bundle. Typically, it takes twelve to fourteen hours to batch anneal one thousand tubes. A major drawback of this process is that individual tubes in the batch do not receive the same time and temperature of heating.
U. S. Pat. No. 4,717,428 to Comstock et al, which is assigned to the same assignee as the present invention, proposes an induction annealing apparatus designed to anneal one tube at a time. The annealing apparatus employs an induction coil mounted inside of an aluminum box which serves as an inert atmosphere chamber. The metal tube is driven through the chamber and the induction coil as inert purge gas is introduced into and discharged from the chamber adjacent the entrance and exit ends of the box. After the annealed portion of the metal tube leaves the induction coil, it travels through an argon purge tube and water-cooled cooling tube also disposed in the chamber defined by the box. Adjustable jaw chucks are located on the entrance and exit ends of the box for supporting the tube as it travels through the chamber. While this apparatus is a step in the right direction, the process is very slow and time consuming and, in addition thereto, uses a considerable amount of argon gas.
Performing the annealing process in an economically viable fashion and in a way to obtain an acceptable quality annealed product, has been unattainable up to this point. If atmosphere is present as the tube is being annealed, the oxygen and nitrogen will react with the tube metal, such as zirconium, causing a layer of discoloration to form on the inside and outside surfaces of the tube. This oxidation residue on the tube is difficult to remove and its presence makes the tube more susceptible to corrosion. If this residue forms, the tube must usually be scrapped.
Consequently, a need exists for improvement in performance of the annealing process so as to obtain metal tubes of more acceptable quality.